1. Field of the Invention
The present invention relates to a fuel control unit and a fuel injection control method for controlling the supply of fuel to the respective cylinders of a multi-cylinder engine and, more particularly, to a fuel injection control art for suppressing the occurrence of abnormal combustion when the operation of an engine shifts from an idling mode to an acceleration mode.
2. Description of the Related Art
Japanese Patent Publication No. 1-3245, for example, discloses an art in which fuel is injected at a different timing from synchronous timing to increase the fuel (hereinafter referred to as "asynchronous injection") in addition to the fuel injection for each cycle to each cylinder (hereinafter referred to as "synchronous injection") when an engine shifts to the acceleration mode. According to the art, as soon as the opening of a throttle valve is detected when the engine shifts from a steady operation mode to the acceleration mode, the amount of the fuel supplied to the engine is increased by the asynchronous injection at a preset pulse width so as to prevent a fuel-air mixture from becoming overlean due to delayed fuel transport in each cylinder of the engine, thereby maintaining good acceleration performance of a vehicle.
In the foregoing conventional art, for each cylinder shifting to an intake stroke after the asynchronous injection is carried out, the injection pulse width for the asynchronous injection is adjusted to be smaller as the time required for the respective cylinders to shift to the intake stroke is prolonged so as to improve the variations in the fuel-air mixture attributable to the difference in the carbureting and atomizing time required for the fuel-air mixture to be taken into each cylinder through an intake port.
As in the case of recent automobiles, however, designing an engine to have a higher compression ratio or setting the idling rpm at a lower value to enhance the vehicular fuel economy is likely to lead to more chances of abnormal combustion or preignition accompanied by a sudden increase in pressure at the time of a shift from idling to acceleration. The abnormal combustion causes the engine to produce a considerably loud abnormal noise, making a driver extremely uncomfortable.
The abnormal combustion inevitably takes place if an intake air temperature is high to a certain degree and an engine having a high compression ratio is running at a low rpm. At this time, it is considered that the fuel-air mixture in a cylinder burns at once, resulting in a considerably greater increase in pressure than that in the combustion based on normal flame propagation.
More specifically, as shown in FIG. 9, the fuel-air mixture in a cylinder is activated to begin chemical reaction at point "a" in the figure as the pressure increases; then, it suddenly reaches a combustion mode at point "b" after the elapse of an ignition delay time. When attention is paid to the ignition delay time from point "a" to point "b" mentioned above, it is found that the ignition delay time becomes shorter as the engine speed decreases. Further, if the air-fuel ratio of the fuel-air mixture is leaner than a stoichiometric ratio, then the ignition delay time tends to be shorter as the air-fuel ratio is richer. Accordingly, in FIG. 9, as the engine speed decreases or as the air-fuel ratio of the fuel-air mixture is richer, point "b" moves closer to an upper compression dead point and the rise in pressure caused by abnormal combustion increases.
Regarding the abnormal combustion and the resulting noise described above, the foregoing conventional art has been presenting a problem in that the asynchronous injection performed at the time of the shift from idling to acceleration causes the air-fuel ratio of a fuel-air mixture to become excessively rich.
More specifically, although the variations in the air-fuel ratio for a cylinder that is not in the intake stroke at the time of asynchronous injection are improved, no considerations have been given to a cylinder that is in the middle of the intake stroke. In the cylinder that is in the middle of the intake stroke when a throttle valve is opened, a fixed amount of fuel is supplied by the asynchronous injection whereas the actual amount of intake air into the cylinder decreases as the opening timing of the throttle valve is delayed. Therefore, as illustrated in FIG. 10, the air-fuel ratio becomes richer as the opening timing of the throttle valve or the acceleration start timing is delayed, and it becomes excessively rich, exceeding a preignition limit at which preignition occurs.
Therefore, a significantly loud noise is produced in such a case where a vehicle starts acceleration after escaping from a traffic jam, and the air that has been warmed in a surge tank is supplied to the engine in a nearly idling mode as the throttle valve is opened and the asynchronous injection is performed at the same time. This is because, in a cylinder that is in the middle of the intake stroke when the throttle valve is opened, the air-fuel ratio of the fuel-air mixture becomes unduly rich, and this is coupled with a high intake air temperature, a high compression ratio, and a low rpm, leading to preignition with a consequent loud noise.
As a solution to the problem set forth above, the amount of the asynchronous injection could be set to a smaller value in order to prevent the air-fuel ratio from becoming too rich; however, doing so would make it impossible to successfully accomplish the original purpose of preventing the deterioration in acceleration performance by increasing the amount of fuel at the beginning of the acceleration of the engine.
Japanese Patent Publication No. 1-32037 discloses the following art. Injection is performed simultaneously according to an injection time, which has been set in the fuel injection valve of each cylinder, when it is determined from a change in the opening of a throttle valve that acceleration has been engaged. For this purpose, the injection time is set so as to provide a lean air-fuel ratio close to but not in a misfire zone for a cylinder in a period between the end of synchronous injection and the end of an intake stroke, and to provide a rich air-fuel ratio close to but not in the misfire zone for the rest of the cylinders. Thus, the air-fuel ratio of each cylinder is controlled to be shifted toward the rich or lean side away from the stoichiometric ratio in accordance with the air-fuel ratio of each cylinder at the time of an acceleration start thereby to prevent knocking.
However, the art disclosed in Japanese Patent Publication No. 1-32037 is not designed to detect the idling rpm of an engine and it is not adapted to determine acceleration after detecting the idling mode of the engine in order to adjust the air-fuel ratio of a first cylinder, in which the filling amount is increased by the release of the throttle valve and the compression pressure substantially increases, to a predetermined air-fuel ratio range. Hence, at the shift from idling to acceleration for a start, too much fuel is supplied for the absolute filling amount of the cylinder which is in the intake stroke and in which the filling amount is increased first, thus leading to a danger of a preignition or abnormal noise as the time of the expansion stroke of the cylinder is prolonged.